Newt Gingrich and Mitt Romney, who have spent much of this year ignoring one another, are now on a collision course in the contest for the Republican presidential nomination, facing a series of state-by-state battles in January that will possibly decide the race by testing which one can best surmount his own weaknesses.

Whatever assumptions may have prevailed a month or two ago have been significantly altered by the surge in support for Gingrich. “There is no longer a scenario where the nomination will default to Romney,” said Steve Schmidt, a Republican strategist who was a top adviser to Sen. John McCain’s campaign in 2008.

Gingrich’s dramatic rise — he now leads national and some state polls, including a new Washington Post-ABC News poll in Iowa — has put serious and unexpected pressure on Romney to adapt and intensify his campaign, which to this point has been both disciplined and unexciting. Steady-as-you-go remains the operative phrase from his Boston headquarters, but other Republican strategists say tactical changes are afoot to deal with the Gingrich threat.

Those changes include a sharper economic message, implicit and explicit contrasts with Gingrich and greater openness with the media. Romney, who has struggled to rally rank-and-file Republicans, is also likely to play on establishment GOP fears about Gingrich as the party’s nominee against President Obama.

Gingrich, in turn, is scrambling to build, almost from scratch, a campaign operation that can match the good fortune of his newfound support. Lacking the funds or the infrastructure of the Romney campaign, Gingrich is looking for a quick infusion of money while urgently adding staff to handle the demands of building organizations in many states at once.

One example of the challenges he faces could be seen early Tuesday evening at his Iowa campaign headquarters in suburban Des Moines. Dozens of cellphones, newly arrived, were being charged in the mostly empty space. Just two volunteers — a mother and grandmother, both from out of state — were making calls. A campaign official said more help is on the way.

Gingrich is trying to hire some former staff members from Herman Cain’s campaign to shore up his organizational efforts in key states. A GOP strategist, who declined to be identified in order to talk openly about the campaign, said Gingrich hopes to roll out endorsements from members of Congress, in part to counter the assertion that he alienated many of his colleagues as House speaker in the 1990s.

The Republican nomination contest could last for months, and some senior Republican leaders say it is still too early to call the competition a two-person race. The answers to both questions will come in January and early February, when five states are scheduled to hold contests: Iowa, New Hampshire, South Carolina, Florida and Nevada. Romney is favored in New Hampshire and Nevada;Gingrich is now the favorite in Iowa and South Carolina.

Of the two campaigns, Romney’s has been preparing far longer for a potentially lengthy nomination battle. But that means Romney cannot allow Gingrich to embarrass him in January. As one strategist put it, “Romney cannot afford to lose two in a row. Thus, Florida is key.”

Romney’s campaign has been building a deep political organization in Florida, particularly in the Miami area, where three Cuban American members of Congress endorsed Romney last week and are mobilizing their networks on his behalf.

But Justin Sayfie, a Florida Republican strategist backing Romney, said the race in Florida will not take shape until the earlier states begin to vote. He said the campaign “needs to make sure the grass-roots organization is in place so we’re ready to chase every vote.”

The same is likely to be the case in South Carolina, where the terrain could easily shift depending on the results in Iowa and New Hampshire.

Gingrich has momentum in Iowa but little organization. Romney has the remnants of his 2008 organization and has been working to mobilize them. But in Iowa, Rep. Ron Paul (Tex.) is also a force, and three other candidates — Texas Gov. Rick Perry, Rep. Michele Bachmann and former senator Rick Santorum of Pennsylvania — are working the state hard.

New Hampshire is Romney’s firewall, but the former Massachusetts governor’s lead there appears to be shrinking. On Wednesday, Romney’s campaign mailed fliers to Republican voters designed to show the broad support he has from the state’s Republican officials. He will return there Sunday for another event.

What gives Romney’s team pause is that New Hampshire voters historically don’t pick a candidate until a few days before the primary. “There’s a high percentage that will wait and think about what Iowa did, and then it’s like, okay, now I have to pay the bill and decide,” Romney adviser Tom Rath said.

Former Utah governor Jon Huntsman Jr. has long hoped to be a spoiler to Romney in New Hampshire, but he has yet to gain significant traction.

Romney advisers see Gingrich as a flawed candidate because of his long record. But with the voting in Iowa less than a month away, some GOP strategists say Romney and his team cannot take count on Gingrich to bring about his own demise, as others in the race have done.

“I don’t think Boston will make any assumption that a candidate will self-destruct,” said Alex Gage, a Republican strategist who was part of the Romney campaign four years ago. “They are smarter than that. They will challenge him.”

Kevin Madden, another ex-Romney adviser who remains an informal member of the team, laid out two possible lines of contrast. Romney has previewed one: the contrast between a Washington politician and someone with a business background. Gingrich has countered by saying Romney has been running for office off and on since 1994.

The other contrast is one of temperament and style. “It’s a choice between teaching America or leading America,” Madden said. “Newt views the job as a lectern of sorts, while the governor is much more focused on a potential presidency that would confront the enormous challenges we face related to the economy, fixing it and getting the nation back on track and creating jobs again.”

Joe Gaylord, who is a longtime adviser to Gingrich but is not part of the campaign team, said Romney will have trouble selling himself to the party’s rank and file. “The problem Mitt has is how to stop the Gingrich rise, and to know him [Romney] is not to love him,” he said. “His closing arguments are no more appealing than his last six years. Therefore, his only option is to go negative, which will backfire.”

Gaylord said that Gingrich needs to keep his coalition motivated to assure that it turns out to vote and that he needs enough money “to remind voters why they like him.”

Gingrich met for two hours Wednesday with a large group of conservatives, fielding a series of tough questions. Amy Kremer, one of the invitees and the chairwoman of Tea Party Express, said Gingrich was asked about past views on climate change and health care and about his rocky personal life, which includes a long-term extramarital affair with his third and current wife, Callista Gingrich.

“People seemed very receptive to what he was saying,” she said. “There were some tough questions that were asked, but he stood up there and answered the questions and appeared to be honest and truthful.”

Mike DuHaime, a top political adviser to New Jersey Gov. Chris Christie, a Romney supporter, summed up the prevailing view among strategists toward Romney’s campaign.

“This is going to be a tough fight,” DuHaime said. “It always was going to be. The wrong thing to do at this point would be to panic and prematurely deviate from the plan.” But he added, “I think they should, and will, begin to contrast with Newt fairly soon to slow his momentum.”